1. Field of the Invention
The present invention relates to a diode mixer, and particularly to a diode mixer used in electronic equipment and microwave-band and millimeter-wave band communication apparatuses for mobile communications and wireless communications.
2. Description of the related Art
There has recently been an increasingly demand for small-sized and high-power apparatuses as communication apparatuses used in a microwave band and a millimeter wave band. With the demand thereof, there has been a demand for a mixer high in frequency conversion efficiency. However, a mixer good in noise characteristic is required simultaneously.
For example, a communication laser device causes the disadvantages that when a mixer is unsatisfactory in noise characteristic, an S/N ratio is reduced and the maximum attainable distance of signal light is decreased, and the cost of a system increases in order to highly maintain the reliability of the system.
In such a system that a millimeter wave band is directly down-converted into a low frequency band, e.g., 100 kHz or the like at which low frequency noise or 1/f noise greatly influences a noise characteristic as in the case of a vehicle-mounted radar or the like, a homodyne system in which an IF frequency of an intermediate frequency signal (hereinafter called “IF signal”) is low, is used at its mixer.
In this homodyne type mixer, the use of a pn diode low in low frequency noise is effective in obtaining a satisfactory noise characteristic.
As a known technique of a mixer, there has been known a known example that has disclosed an even number harmonic mixer using an antiparallel diode pair (refer to, for example, Japanese Patent Publication No. 2795972, the 2nd to 3rd sections and FIG. 6, and the 4th to 5th sections and FIG. 1).
As a known technique of an electronic high frequency switch, there has been disclosed an example wherein a coil is parallel-connected to a diode, and the resonance frequency of a resonant circuit constituted of a reverse bias capacitance of the diode and the coil is set to be close to the frequency of a used high frequency signal to increase impedance at near the used frequency, thereby improving isolation (refer to, for example, Japanese Patent Laid-Open No. Sho 55(1980)-42412, the lower left-hand section in the 54th page and FIG. 6).
As a known technique of a diode switch circuit, there has been disclosed an example wherein a reactance circuit including a series connection circuit configured of an inductance and a capacitor is connected across a diode in parallel with the diode, whereby a diode switch circuit superior in high frequency characteristic is configured (refer to, for example, Japanese Patent Laid-Open No. Sho 61(1986)-61524, the lower left-hand section of the 108th page and FIG. 1).
As a known technique of a λ/4 type switch circuit, there has been disclosed an example wherein a circuit in which an inductor having an inductance value that produce parallel resonance with the capacitance of a diode and a DC-cutting capacitor are connected in series, is connected in parallel with the diode(refer to, for example, Japanese Patent Laid-Open No. Hei 2(1990)-108301, the lower right-hand section of the 2nd page and FIG. 1).
As a known example of a frequency converting circuit, there has been disclosed an example wherein coils are series-connected to their corresponding diodes that constitute an antiparallel diode pair of an even number harmonic mixer to compensate for junction capacitances of the diodes, thereby reducing a loss at frequency conversion (refer to, for example, Japanese Patent Laid-Open No. 2004-140438 and FIG. 1).
However, the mixer using the pn diode was reduced in frequency conversion efficiency in some instances due to the existence of a diffusion capacitance in the diode.
An equivalent circuit of the diode is represented as one in which a variable resistive component and a variable capacitive component are connected in parallel.
And the capacitance of the pn diode is given as expressed in the following equation (1):Cj=(Cjo/(1−(V/Vf))M)+((qτIs)/(nkT))exp(qV/(nkT))  (1)
where Cjo indicates a capacitive component at 0V, Vf indicates a built-in voltage, M indicates a tilt coefficient, q indicates an elementary charge, τ indicates transition time, Is indicates a saturation current, n indicates an ideal coefficient, and k indicates the Boltzmann constant, respectively.
The first term on the right side of the equation (1) indicates a junction capacitance, and the second term thereof indicates a diffusion capacitance.
A diffusion capacitance that changes in the form of an exponential function with an anode voltage exists in the capacitive component of the diode in addition to the junction capacitance. Thus, a region that increases exponentially with an increase in anode voltage exists in the capacitive component of the diode.
When power of a local oscillation signal (hereinafter called “LO signal”) increases in the diode mixer using the pn diode, that is, when the amplitude of the anode voltage increases, the capacitive component of the diode increases exponentially in the vicinity of its built-in voltage (or on voltage or threshold voltage) at forward biasing in accordance with the second term of the equation (1).
On the other hand, on the mixing, nonlinearity of a current-voltage characteristic in the vicinity of the built-in voltage is important. When the anode voltage amplitude of the LO signal reaches a region in which a current in the vicinity of the built-in voltage changes most abruptly, conversion gain becomes the largest and decreases sharply at its anode voltage amplitude slightly lower than this region.
Therefore, a problem arises in that even though attempts are made to increase the power of the LO signal, increase the anode amplitude voltage of the LO signal and raise conversion gain for frequency conversion, the capacitive component of the diode increases exponentially and the power of the LO signal is not effectively inputted to the variable resistive component of the diode, which contributes to the frequency conversion at a mixer circuit, so that satisfactory frequency conversion efficiency is not obtained.